In the field of motor vehicles which are operated by means of combustion engines it is a general requirement that the concentration of harmful substances in the engine's exhaust gas should be as low as possible. These harmful substances are mainly present in the form of unburnt residues of hydrocarbons (HC), oxides of nitrogen (NO.sub.x) and carbon monoxide (CO). In today's motor vehicles equipped with gasoline engines, a purification of the exhaust gas is normally carried out by means of a catalytic converter, or catalyst, arranged in the exhaust system. In the modern so-called three-way catalyst, the major part of the above-mentioned harmful substances is eliminated by means of various well-known catalytic reactions.
Today's catalysts provide a very high degree of purification, i.e. a conversion of harmful exhaust gas components to carbon monoxide and water. This means that they eliminate a very high quantity of the harmful emissions in the exhaust gas at the appropriate operating temperature of the catalyst. However, they suffer from the problem that they must be heated for a certain time period in order to reach the operating temperature at which an optimum degree of purification can be obtained. The so-called "light-off temperature" of the catalyst is approximately 200-350.degree. C. and can be defined as the temperature at which the catalyst provides a 50% degree of purification of a certain harmful component in the exhaust gases. During the initial warm-up phase of the catalyst, which is approximately 30-90 seconds, the catalyst cannot operate in an optimum manner as regards the elimination of the harmful substances in the exhaust gases. Obviously, this constitutes a problem which arises during cold starting of a vehicle.
A possible way to reduce the quantity of harmful emissions during said initial warm-up phase is to carry out various measures in order to shorten the time taken for the catalyst to reach its light-off temperature. During a cold start, this can be achieved by generating increased heat energy into the exhaust system which subsequently causes the catalyst to be rapidly heated.
A previously known arrangement for obtaining this reduction in time for the light-off temperature to be reached is one comprising an electrically heated catalyst which is arranged upstream of the main catalyst. However, this arrangement implies certain drawbacks. Firstly, the cost for a heatable catalyst is considerable. Furthermore, the consumption of electrical energy is relatively high. An additional power supply such as an extra battery may be required in the vehicle. Also, the durability of the electrically heatable catalyst may constitute a problem.
Another arrangement, which is disclosed in the journal Motortechnische Zeitschritt, Vol. 55 (1994), No. 4, pages 198-206, "Die Motoren im neuen Opel Omega", Heinz-Ewo Brand at al, comprises means for injecting secondary air into the exhaust gas. This secondary air is mixed with the exhaust gas in the exhaust port and in a plenum volume immediately downstream of the engine's exhaust valves, resulting in an oxidation of the mixture consisting of the exhaust gases and the secondary air. This oxidation results in a generation of heat energy which is fed to the catalyst, which consequently will become heated.
This arrangement is based on the fact that, during cold starting, the engine is operated so as to provide a certain stoichiometric excess of fuel in the air/fuel mixture which is fed to the engine. The enriching of the air/fuel ratio to a level which gives a lambda value below .lambda.=1 will cause excess hydrogen (H.sub.2), carbon monoxide, (CO) and hydrocarbons (HC) to be generated in the exhaust gases. By reducing .lambda. to a value of approximately 0.7 for example, the corresponding amount of hydrogen in the exhaust gases will then be present in an amount of about 5% by volume of the exhaust gas.
Such low values of the A parameter can be achieved by altering the engine's control system in such a way that the control output to the fuel injector is arranged to ensure a rich air/fuel mixture during the start-up phase. Such may occur by increasing the fuel injection time and/or decreasing the amount of input air to the engine. Further methods are also available for increasing the amount of hydrogen gas and other combustible components in the exhaust gas, such as changing fuel injection or ignition timing, adjusting the timing of the engine valve lifting or even by appying stratified combustion in the combustion chamber.
As previously mentioned, the secondary air is mixed with the exhaust gases, resulting in an oxidation process which is mainly due to the hydrogen which is present in the exhaust gases. The oxidation reaction generates a high amount of heat energy which is guided through the exhaust pipe and to the catalyst, which subsequently becomes rapidly heated.
Although the above-mentioned arrangement results in a reduced time taken until the light-off temperature of the catalyst is reached, it suffers from the drawback that the performance of the engine Is negatively affected. This is due to the fact that the known arrangement cannot be effectively tuned for optimum engine power and engine torque. As regards the tuning of an engine, the design of the engine outlets should always be considered, so as to provide an optimum volumetric efficiency of the engine. The geometry of the exhaust manifold according to the arrangement described in the above-mentioned article "Die Motoren im neuem Opel Omega" does not allow any such tuning, which is essential if the engine's performance is to be optimized.
Another drawback of the arrangement according to said article is that during cold starting, little or no consideration is taken to the temperature of the catalyst. This means that the engine is controlled so as to generate hydrogen when this is not needed, for example when the catalyst has already reached its light-off temperature. In other words, excess fuel can be supplied to the engine, which results in increased fuel consumption.
For turbocharged engines the time period to reach optimum performance of the catalyst is usually longer than for naturally aspirated engines. This is mainly due to the fact that the turbocharger acts as a heat sink and reduces the temperature of the exhaust gas before entering the catalyst. The above-mentioned arrangements for exhaust oxidation have as yet not been applied to turbocharged engines.
Another system for reducing the emissions is known from the documents WO-A-92/22734 and WO-A-93/07365, which disclose a system in which the hydrogen and oxygen mixture is guided to a separate afterburner combustion chamber, which is arranged downstream of the exhaust pipe. When the exhaust gas reaches the afterburner combustion chamber it is ignited by means of a special ignition device immediately after the engine has first fired. This is achieved by ensuring that the concentration of hydrogen and oxygen remains within known flammability limits. In order to obtain the required concentrations, the fuel/air mixture is enriched significantly so as to obtain additional hydrogen, whilst additional oxygen is added by means of a supplementary air pump.
Although an improvement is obtained hereby, a severe drawback of the system is that an ignition device is required in the afterburner in order to ignite the gas mixture. Such an ignition device constitutes an extra component which is prone to failure. Moreover, from the consumer point of view, this is undesirable due to the resultant extra cost involved with the more expensive exhaust system and the ensuing costs of servicing and/or replacing worn-out or faulty afterburner ignition devices.